The present disclosure relates to carbonated fly ash and a method of making the same. The present disclosure further relates to the use of the carbonated fly ash to control the setting behavior of cement slurries.
During the combustion of coal, a secondary by-product commonly referred to as “fly ash” is obtained. Fly ash typically comprises fine powders that contain polycrystalline particles generally having spherical, hexagonal and tubular shapes, with sizes ordinarily ranging from 1-100 μm range (ca. 30%) as well as amorphous particles (ca. 70%). They are lightweight materials and possess a complex chemical composition generally comprising silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO), sulphates (e.g., Na2SO4) and other small amounts of oxides and minerals formed from a combination of these. Fly ash is commonly classified based on its calcium oxide content: Class C (ca. 15-30% CaO), Class F (ca. 5-10% CaO) and Class I (ca. 10-20% CaO). The use of fly ash is a function of its chemical composition and microstructure. Upon hydration, some of the fly ash particles react very fast and others are chemically inactive for longer period of times (more than two years). Class F fly ash is generally not cementitious, but has pozzolanic properties, in that it can undergo a cementitious reaction when mixed with water and free lime or calcium hydroxide. In contrast, due to its increased calcium oxide content, Class C fly ash, in addition to pozzolanic properties, also has cementitious properties, in that it can undergo a cementitious reaction when mixed with water.
There is a national demand to protect the environment, conserve energy, and reduce costs of disposal of the secondary materials resulted from power plants, as well as find them new economical applications. Additionally, the large quantities of fly ash that are collected during the combustion of coal can add undesirable costs to the production of electricity due to the environmental concerns associated with the fly ash's disposal. Accordingly, due to its pozzolanic and/or cementitious properties, there have been substantial efforts to increase the use of fly ash in industries such as the well construction, road construction, and building construction industries. For instance, fly ash has been proposed as a low cost additive for downhole cements in well construction. However, because the properties of fly ash when set are generally not sufficient for many applications, it is common to mix cement and fly ash. Due to its low calcium oxide content, Class F fly ash may have the role of inert filler when mixed with cement. Class C fly ash, however, can be activated and become cementitious once added to the cement slurry.
The addition of fly ash to cement slurries results in the modification of its physical and mechanical properties including, workability, strength, shrinkage, porosity, and permeability. However, it is desirable to understand the chemical role of fly ash and to find ways to control and tune its properties in order to obtain the response desired. In particular, methods are sought of extending the setting time as well as providing enhanced stability during the induction period of cement hydration. Stability during the induction period of cement hydration is desirable during well cementing applications where it is necessary to pump a specific viscosity cement slurry over large distances within a specific time, without changes in viscosity that would undesirably effect pump rates and the like. Extension of the setting time may be particularly desirable in deep-well applications where set retarding additives, such as lignosulfonates, are commonly included in the cement slurries.